In a BWR, the high-temperature (.about.288.degree. C.) water coolant is highly oxidizing due to dissolved radiolytically produced chemical species, such as oxygen and hydrogen peroxide. These molecules and/or compounds are generated as water passes through the reactor core and is exposed to very high gamma and neutron flux levels. Because of the resultant high electrochemical potential ("ECP"), reactor structural materials, such as stainless steels and nickel-based alloys, can suffer stress corrosion cracking ("SCC").
It is well known that SCC occurs when oxygen is present in the reactor water in concentrations of about 5 ppb or greater. Stress corrosion cracking is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the ECP of metals. Electrochemical corrosion is caused by a flow of electrons from anodic and cathodic areas on metallic surfaces. The ECP is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
The useful lifetime of reactor components, such as piping and pressure vessel internal structures, can be limited by SCC. To date, SCC has resulted in a large inspection and repair cost in the nuclear industry and could eventually lead to premature decommissioning of BWR plants due to economic considerations.
A number of countermeasures have been developed to mitigate SCC in BWRs by sufficiently reducing either the stress level, the material susceptibility to cracking, or the "aggressiveness" of the environment. Of the various mitigation strategies, reducing the environmental aggressiveness (i.e., oxidizing potential) can provide the broadest, most comprehensive approach, since the environment contacts all the potentially susceptible surfaces of interest. The primary strategy to reduce the ECP of the water to some benign value has been to add hydrogen gas to the reactor feedwater in sufficient quantity that hydrogen is available to chemically recombine, in the presence of a radiation field, with dissolved oxygen and hydrogen peroxide to form water. This process is called hydrogen water chemistry (HWC).
If the hydrogen concentration is sufficient, the resultant ECP can be reduced below the SCC threshold value. As used herein, the term "threshold value" means a corrosion potential at or below a range of values of about -230 to -300 mV based on the standard hydrogen electrode ("SHE") scale. Stress corrosion cracking proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower rate in systems in which the ECP is below the critical potential. Water containing oxidizing species such as oxygen increases the corrosion potential of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in corrosion potentials below the critical potential.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2 and O.sub.2. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. This concentration of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 is oxidizing and results in conditions that can promote SCC in susceptible structural materials.
When HWC is used to make the bulk coolant sufficiently reducing, the nitrogen isotope .sup.16 N, which is normally present in the water phase during reactor operation, partitions into the steam phase as it is reduced from a nonvolatile form (a highly oxidized form such as nitrate or nitrite) to a volatile form (a less oxidized form such as NO and eventually to a highly reduced form such as NH.sub.3). This results in an increase in .sup.16 N gamma activity in the steam lines and turbine systems, which can exceed regulatory personnel radiation exposure limits at hydrogen addition levels needed for broad SCC protection. Feedwater hydrogen additions, e.g., of about 200 ppb or greater, that reduce the corrosion potential below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived .sup.16 N species, as shown in FIG. 2. For most BWRs, the amount of hydrogen addition required to provide mitigation of SCC of pressure vessel internal components results in an increase in the main steam line radiation monitor ("MSLRM") by a factor of greater than about four.
To reduce .sup.16 N gamma activity to acceptable levels at these plants, it is now necessary to add shielding at strategic locations, which can be costly. In most cases, this consideration has limited use of HWC to protection of only those components where the ECP can be reduced below the SCC threshold without a significant increase in steam-phase .sup.16 N.